I'm building a PA subwoofer using an Eminence Sigma Pro-18" (see other thread), and I want to protect it with a fuse. The driver handles 500w/RMS, and the DC resistance is 6.4 ohms. There is no crossover because I am bi-amping. How can I figure out what fuse rating to use, and should it be slo-blow or fast-blow?

Thanks,
Evan

Evan, assuming the driver will truly handle 500w RMS long term, then the fuse rating can be calculated as follows:
The current drawn at 500W into 6.4ohms is I=sq root (W/6.4)
This works out at 8.8 amps, so to be on the safe side try a 7.5 amp slow blow fuse.

Remember that a fuse on a speaker will always blow after a time. It can get very annoying(trust me) when it's come to change it and you don't have some.

Hmm..I was not aware of this problem. Maybe I'll nix the idea, or keep an emergency wad of aluminum foil on hand! How long will a slow-blow fuse usually last in this application?

Evan

hehe aluminium foil. I have alot of experience with aluminium foil. It's actually depend. I ahve see case when they just didn't last a week. It depend if you push your system hard. Also playing with aluminium foil is not very reccomended. You will always have a problem when it's they foil in place.

You can also use a higher value fuse but passed a point it's going to be the speaker protecting the fuse(tweeter make excellent fast acting fuse btw :).

Why not use a fast auto-fuse (the kind with a small button to press when you've removed the load on the speaker)? They usually last more than 1000 times, before you need to replace them.

//magnus

but those are usualy rataed att above 100amps...?

Not the ones I mean.

They look like:

<img src=http://www.elfa.se/images/ct_tnail/A10672_C.JPG>

http://www.elfa.se/images/ct_tnail/A10672_C.JPG

And they are rated from 2 A and up.

//magnus

That's a circuit breaker. It may affect the sound.

Ok, never mind. ;=)

I thought they'd do.
But I've seen similar fuses on older Yamaha speakers.

//magnus